An interesting paper has been published in the Vision Development and Rehabilitation Journal. The paper is all about the posterior parietal lobes, which are fed visual information, from the occipital lobes by the dorsal stream. As we explained in our What is CVI? section, the brain receives visual information from the eyes. The first stop is the occipital lobes where the brain provides colour, contrast and clarity. This information then travels along the dorsal stream to the posterior parietal lobes where the brain gives that visual information depth, and maps it, like an extraordinary architect, at the same time as we are moving through it.
Visual challenges from this part of the brain are called dorsal stream dysfunction.
This paper features several case studies that are very different, yet all have cerebral visual impairments, showing how varied it is in both its presentations and its effects. The paper also suggests how common dorsal stream dysfunction is. However, because it is both difficult to understand and observe, it is likely that many people go undetected and undiagnosed.
The paper starts with a clear explanation of the posterior parietal lobes, and what they do, then leads into a series of the individual case studies. The case studies are ordered to build up an understanding of how the posterior parietal lobes work, and what happens when they work differently.
In the first case, a person referred to as MC, the person's occipital lobes have been damaged and they produce no image at all, however the parts of the brain that process movement (middle temporal lobes) and map the visual scene (posterior parietal lobes) were both intact. We linked to a short clip of biological motion in our movement section - can you imagine what it might be like to process the movement and the depth of your surroundings, without the picture?
Imagine walking through this mall with no image, just visual information from movement and your internal three-dimensional map making facility. We know that MC can confidently navigate different environments, avoiding obstacles and not bumping into things, even though the brain creates no picture. This is called blindsight.
Now try to imagine walking through the same mall without that map, but with the picture, as with the second case study, Sergeant K, who retained his occipital lobes but had a profound injury to his posterior parietal lobes. For him, everything is bright and clear, all colours defined, but there was no known depth and he couldn't judge distances. With this brain injury, reaching for the silver handrail on the left of the picture, you may well miss, possibly causing a stumble or fall. Walking through the mall, as people pass you, you may well bump into them, or brace yourself fearing collision.
Which do you imagine is the more challenging?
As you read through the different case studies, imagine walking through the mall looking through their eyes and understand the challenges they face. To understand CVI it is critical that we can do this, because the affected person can't, it is non-conscious.
The different case studies had different strategies, however several relate to reducing visual clutter. Tom (case study 12), looked around for the first time age seventeen, when in a bright tent. A further exciting technique was adopted for Lea (case study 11), who underwent an intensive programme to reduce her simultanagnostic vision, with great success.
When you have a visual impairment, there are three key options when faced with any challenges:
With visual challenges from the posterior parietal lobes, including simultanagnostic vision, optic ataxia and apraxia of gaze, this paper suggests that there is an enormous potential to learn.
Posterior Parietal Visual Dysfunction: An Explanatory Review (page 10)
Dutton G, Chokron S, Little S, McDowell N
Vision Development & Rehabilitation
Volume 3 Issue 1 April 2017
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